Present optical systems for fundus imaging, i.e., systems for delivering optical beams onto the fundus, include: (a) laser beam coagulation apparatus in which a laser beam is focused onto tissue in the posterior chamber of an eye to raise the temperature of the tissue to a high enough temperature to cause thermocoagulation and (b) Optical Coherence Tomography ("OCT") apparatus in which a low coherence optical beam obtained, for example, from a superluminescent diode CSLD) is focused onto the fundus and a cross-sectional image of the fundus is obtained using short-coherence interferometry. U.S. Pat. No. 5,067,951 discloses an example of a laser beam coagulation apparatus and an article entitled "Optical Coherence Tomography" by D. Huang et al., Science, 254, Nov. 22, 1991, pp. 1178-1181 discloses an example of an OCT apparatus.
The above-identified apparatus typically utilize an optical set-up which is comprised of a slit lamp and an ocular lens. In operation, illumination is provided when the ocular lens, in conjunction with the lens of an eye, forms an aerial fundus image onto the focal plane of the slit lamp. The aerial fundus image is observed through an observation path of the slit lamp optics. In addition, an optical beam is coupled into the slit lamp observation path by a beamsplitter. In these apparatus, the ocular lens is normally held by hand close to a patient's eye or in direct contact with the cornea.
In the slit lamp and ocular lens optical set-up which is typically used in the above-described apparatus, reflection of an illumination beam and an optical beam from the cornea and from the ocular lens is much more intense than reflection from the fundus. For example, reflectivity of the fundus is approximately 10.sup.-4 whereas reflectivity of the cornea and reflectivity of a typical ocular lens (for example, a Volk double aspheric bio lens manufactured by Volk of 7893 Enterprise Drive, Mentor, Ohio 44060) are both on the order of 4%, which value of reflectivity is much greater than the reflectivity of the fundus. Consequently, the quality of a fundus image is degraded by artifacts which result from reflections from the ocular lens and the cornea. In order to remove such artifacts, it is necessary to prevent reflections from the cornea and from the ocular lens from entering the observation path. Present attempts to prevent reflection from the ocular lens from entering the observation path entail tilting the ocular lens with respect to the illumination beam and the optical beam. However, tilting the ocular lens is not satisfactory because it introduces astigmatism and vignetting. In addition, attempts to prevent reflections from the cornea from entering the observation path entail using a contact ocular lens. However, using a contact ocular lens is not satisfactory because it is difficult to eliminate reflection from both the cornea and the ocular lens when a hand-held ocular lens is used. As a result, it is difficult to obtain a good fundus image in apparatus using this type of optical set-up.
Attempts have been made to solve the above-described problem by geometrically separating reflections from the ocular lens and the cornea by using: (a) ring illumination (ring illumination is obtained, for example, by placing a stop in the illumination path to remove the center of the illumination beam) and (b) a small, centered aperture for observing the fundus to geometrically separate reflections from the cornea and the ocular lens. However, using ring illumination is not satisfactory because reflection of the optical beam from the cornea and from the ocular lens cannot be avoided, except if the optical beam diameter is small enough so that it can be coupled into the eye, off-center. However, this is disadvantageous because it requires a large pupil diameter to avoid optical beam vignetting.
In light of the above, there is a need in the art for an ophthalmologic apparatus which: (a) has a simple structure; (b) delivers an optical beam onto the fundus without vignetting; and (c) provides a high quality fundus image.